Exoplanet HD189733b

In an attempt to find out whether it was possible at the Tinyblue Observatory to detect extra-solar planets (exoplanets), I started with one of the easiest ones, a star in the constellation of Vulpecula designated HD189733. In 2005, French astronomers discovered a planet the size of Jupiter “transiting,” or passing across the face of this star. The star, nearly 63 light-years away is just slightly smaller than our sun. The orbital period of the planet, designated HD189733b is 2.2 days.

The star is relatively bright at magnitude 7.67 and its apparent brightness drops by 0.028 mag, or about 0.4% during the transit of the planet. The duration of the transit, from when the planet first starts to move across the face of the star, to when it leaves is 109.6 minutes.

These characteristics suggested that I would have a good chance of observing the dip in brightness during a single night of observation if I happened to have a clear night on which a transit would begin in the evening after dark. Fortunately, the Exoplanet Transit Database website provided the necessary forecast:


It was in a good position in the sky for viewing from midnight until 4:00am on Saturday. I set up the telescope to track the star and had the camera take 30-second exposures continuously through the night. The exoplanet is about 13% larger than Jupiter, but much closer to its parent star. It revolves around the star in only 52.8 hours!

The fact that is about 62.9 light-years from Earth means that the light I was recording had been emitted from the star right around the day I was born in 1947. Auspicious, perhaps?

Details of the observation program are described in the Tinyblue Observing Plan 2010-06-11. In a reference frame of the surrounding star field, the program star is identified as V and several comparison stars C1, C2, etc are indicated. Accurate magnitudes of the comparison stars have been published and can be used as reference to determine the magnitude of the program star.

A light curve for the transit event clearly demonstrates the dimming of star HD 189733 as light is blocked by exoplanet HD 189733b.

Light curve for transit of HD189733b

During the night, we acquired 328 images of the star. The images were “reduced” (cleaned up using special reference frames) and the star magnitudes measured using photometric software. Results were compiled in an Excel spreadsheet, HD189733b_Multi-Image Photometry_ExtraDecimals and a graph of magnitude vs. time was generated.

Click for a full-size version of the Light Curve of HD189733

In the graph of our results, the rapid fluctuations are due to random effects in the atmosphere and in the measurement equipment. But ignoring this “noise” in the signal, it is still clear that there is a definite drop in brightness of about 0.03 magnitude or 0.4%.

More information about the exoplanet is at http://en.wikipedia.org/wiki/HD_189733_b